The present invention relates generally to equipment utilized and operations performed in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides positional control for downhole actuators.
A pressure actuated downhole actuator is typically operated by applying pressure to a line in order to displace a piston of the actuator. However, some well tools, such as downhole chokes and other types of flow control devices, are operated using a type of actuator in which the piston is not just required to displace, but is also required to displace a certain distance or to a certain position in order to produce a desired change in the well tool. For example, a certain displacement of the piston may produce a corresponding change in flow rate through a downhole choke.
Unfortunately, pressure is generally applied to an input line of the actuator from a remote location, such as a surface location, which may be thousands of meters from the actuator. Fluid compressibility, friction, expansion of the input line due to applied pressure, thermal expansion of the input line and fluid, etc. cause it to be very difficult to determine how the piston displaces in response to pressure applied to the input line.
Various methods have been devised for overcoming this problem, but each of these methods has its own shortcomings. One method is to use a displacement sensor in the actuator to directly sense the movement of the piston. However, this method requires that the sensor be accommodated in the well tool, and that a communication system be provided for transmitting signals from the sensor to the surface. In addition, the sensor must be capable of withstanding the downhole environment (high temperatures/pressures, vibration, etc.).
Another method is to use a certain number or pattern of pressure applications to the input line to produce a corresponding displacement of the piston. However, this method requires that the well tool be designed with a control system capable of decoding the pressure applications, and that an operator at the surface be capable of determining when the appropriate pressure applications have been received and decoded at the control system. The more complex the control system, the less likely that it will survive long term in the downhole environment.
Therefore, it may be seen that improvements are needed in the art of positional control of downhole actuators. Preferably, systems and methods for controlling the position of a piston in a downhole actuator should be reliable and relatively inexpensive, but should provide for very accurate control of position.